In a third Generation Partnership Project (“3GPP”) Long Term Evolved (“LTE”) system without a Carrier Aggregation (“CA”) technique, such as LTE Frequency Duplex Division (“FDD”) or Time Duplex Division (“TDD”) systems with a (positive) Acknowledgment (“ACK”)/Negative Acknowledgement (“NACK”) bundling mechanism, one Uplink (“UL”) feedback represents a HARQ feedback of a single DL subframe or bundling window. If a Physical Downlink Control Channel (“PDCCH”) has not been successfully detected by a UE, the UE may determine presence of a Discontinuous Transmission (“DTX”) and thus will not transmit a HARQ feedback on a Physical Uplink Control Channel (“PUCCH”) or a Physical Uplink Shared Channel (“PUSCH”). If the PDCCH has been detected, an ACK or a NACK will be reported on the PUCCH or PUSCH based on whether a Physical Downlink Shared Channel (“PDSCH”) has been successfully decoded or not. In this manner, it is possible for an evolved Node B (“eNB”) to detect the PDCCH DTX by measuring the power level of the HARQ feedback on the PUCCH or PUSCH.
As is known to those skilled in the art, the CA technique is introduced in the 3GPP LTE Release 10 (“Rel-10”) with an aim of doubling the peak throughput by aggregating multiple carriers to perform simultaneous transmissions for a single UE. In a CA system with multiple Component Carriers (CCs) in a DL, multiple parallel HARQ transmissions may be performed for one CA-enabled UE in a Primary Cell (“PCell”) and activated Secondary Cells (“SCells”). Thereby, the UE is required to report several HARQ feedbacks for the PCell and SCells through a single UL subframe. To this end, new HARQ feedback modes of a PUCCH format 1b with channel selection and a PUCCH format 3 have been specified by the 3GPP for the CA, which are capable of carrying more feedback bits for multiple HARQ transmissions in one DL subframe or bundling window.
However, in the CA with ACK/NACK multiplexing, one UL HARQ feedback represents ACK/NACK for more than one DL HARQ transmissions, such as 2 CCs in DL. In particular, the UE will transmit the HARQ feedback on the PUCCH or PUSCH as long as one DL grant is received in a corresponding subframe or bundling window. A NACK/DTX may also be reported by the UE for subframes that failed to detect the PDCCH (resulting in DTX) or decode the PDSCH (resulting in NACK). As such, whether the NACK/DTX represents the NACK or DTX becomes ambiguous, and the eNB cannot distinguish the NACK against DTX from the reported ambiguous feedback NACK/DTX.
For example, in case of the CA FDD or TDD system with a bundling window size (M) being equal to 1, if an ACK or a NACK is reported from the UE for one of paired Codewords (“CWs”), it may be assumed that there is no PDCCH being lost. However, if the NACK/DTXs are reported for both CWs, the eNB may get confused about these ambiguous feedbacks. The ambiguity is more serious in the CA TDD system with M>1, where there is more ACK/NACK multiplexing and bundling within the bundling window and among CWs. Since the NACK/DTXs are bundled for paired CWs, any received NACK/DTX has an uncertainty issue with respect to the NACK or DTX.
The uncertainty of received NACK/DTX brings problems for normal HARQ retransmissions and outer-loop adjustment of PDCCH Link Adaptation (“LA”). Normally, the HARQ retransmission and outer-loop adjustment of the PDCCH LA are performed according to the received feedback as follows:
1) Received NACK (“RV”) probability as Block Error Ratio (“BLER”) target of PDSCH LA)
The next Redundancy Version (“RV”) is retransmitted until an allowed maximum number of HARQ transmissions. The overall RV sequence is RV0→RV2→RV3→RV1 as recommended by 3GPP. The NACK is used for outer-loop adjustment of PDSCH LA to guarantee 10% BLER target.
2) Received DTX (1% probability as targeted PDCCH loss rate of PDCCH LA) The previous RV is retransmitted. The DTX is used for outer-loop adjustment of PDCCH LA to guarantee 1% PDCCH loss rate.
From the above, it can be seen that the HARQ retransmission and PDCCH LA outer-loop adjustment depend heavily on the definite feedback of the NACK or DTX; otherwise, the eNB does not know how to perform the HARQ retransmission and PDCCH LA outer-loop adjustment, which might result in a loss of HARQ combining gain and inaccuracy of PDCCH LA.
Generally, there are two solutions to handle NACK/DTX ambiguity. One is handling the NACK/DTX simply as the DTX and another one is handling it simply as the NACK. However, both solutions result in undesirable and adverse impacts on the system performance. Regarding handling as the DTX, given 10% PDSCH BLER target and 1% PDCCH lost rate, the NACK/DTX is likely to be the NACK with more than 90% probability. However, if it is simply handled as the DTX, a previous RV is retransmitted. In this way, only Chase Combing (“CC”) can be done by the UE and Incremental Redundancy (“IR”) combining gain will be lost, which degrades the CA throughput, especially in case of channel with fluctuation. Further, since the real PDCCH DTX probability is about 1%, the estimated DTX probability with this method is too high (about 10%) to be used for outer-loop adjustment of PDCCH LA. In other words, there exists a false alarm of the PDCCH DTX, which may lead to an improper outer-loop adjustment. Regarding handling as the NACK, it may give rise to more HARQ transmissions if the ambiguous NACK/DTX actually means the DTX. For instance, if the initial RV0 has been lost and the UE transmits the NACK/DTX as the feedback to the eNB, the eNB should retransmit the RV0 for improving the success rate of the decoding at the UE side by setting the NACK/DTX as the DTX. However, due to handling the NACK/DTX as the NACK, the eNB would retransmit the subsequent RVs (e.g., RV2, RV3 and RV1), which likely results in a decoding failure at the UE side due to the absence of RV0. Thus, extra HARQ retransmissions are introduced resulting in throughput degradation. Further, due to simply handling as the NACK and miss detection of the PDCCH DTX, the real PDCCH DTX cannot be detected and thus abnormal PDCCH LA may arise since a proper determination of the DTX plays a key role in the outer-loop adjustment of the PDCCH LA, as mentioned above.
In short, simply handling the NACK/DTX as either NACK or DTX is not optimized for both the HARQ transmission and the LA outer-loop adjustment.